Merge branch 'dvc-virtual-has-value'

Allow overriding wxDataViewModel::HasValue() to specify which cells
should, and should not, show anything.

Closes https://github.com/wxWidgets/wxWidgets/pull/1792

Closes #18724.

include/wx/dataview.h

@@ -208,7 +208,7 @@ public:
     // return true if the given item has a value to display in the given
     // column: this is always true except for container items which by default
     // only show their label in the first column (but see HasContainerColumns())
-    bool HasValue(const wxDataViewItem& item, unsigned col) const
+    virtual bool HasValue(const wxDataViewItem& item, unsigned col) const
     {
         return col == 0 || !IsContainer(item) || HasContainerColumns(item);
     }